Method of making a semiconductor integrated circuit device utilizing simultaneous outdiffusion and autodoping during epitaxial deposition

ABSTRACT

A semiconductor integrated circuit device includes circuit elements having relatively different performance characteristics in which buried regions having different chemical elements are used to autodope an epitaxial layer to different degrees.

This invention relates to a semiconductor integrated circuit devicewhich includes circuit elements having relatively different performancecharacteristics and to a method of manufacturing these devices.

Bipolar digital and linear integrated circuit devices are often formedin a layer of semiconductor material which is usually an epitaxial layeron a substrate which contains buried regions of the same typeconductivity as the layer but with a relatively low resistivity. Whenlow voltage, high speed circuit elements, such as those required fordigital circuits using integrated injection logic (I² L), are to beformed, it is desirable for the epitaxial layer to be relatively thinand to be relatively highly doped, i.e. to have relatively lowresistivity. On the other hand, when high voltage circuit elements areto be formed, it is desirable to have a relatively thick, relativelyhigh resistivity layer. When it is desired to combine relatively lowvoltage and relatively high voltage circuit elements on the same chip,compromises are required in the thickness and the resistivity of theepitaxial layer.

Manufacturers who use established technology for the manufacture ofbipolar integrated circuits (usually requiring high voltage circuitelements) have encountered a problem when attempting to integrate lowvoltage I² L circuit elements into their devices. If no modifications ofthe processing are made, the resulting I² L elements have a poor ratioof inverse beta to forward beta because of poor emitter efficiency. Toimprove the inverse beta of these devices attempts have been made toincrease the forward beta but this has resulted in poor yields.

In the drawings:

FIG. 1 is a partial diagrammatic cross section of an example of anintegrated circuit device illustrating the principles of the presentinvention;

FIGS. 2-4 are a series of partial cross-sectional views illustratingsteps in the present method.

An example of an integrated circuit device 10 illustrating theprinciples of the present invention is partially and diagrammaticallyillustrated in FIG. 1. The integrated circuit device 10 includes a body12 of semiconductive material, usually silicon, including a substrate 14of one type conductivity and an epitaxial layer 16 of opposite typeconductivity with an interface 18 therebetween. In usual practice, andin this example, the substrate 14 is of P type conductivity and theepitaxial layer 16, as formed, is of N type conductivity.

Adjacent to the interface 18 and in the substrate 14 is a firstlocalized region or buried pocket 20 which contains opposite typeconductivity modifiers to a relatively high degree. As shown in FIG. 1,the buried pocket 20 extends somewhat beyond the interface 18 into theepitaxial layer 12. While this is diagrammatic, it generally representsthe condition in finished devices which results from out-diffusionand/or autodoping of modifiers from the substrate into the epitaxiallayer during its growth.

A second localized region or buried pocket 22 of opposite typeconductivity is in the substrate 14 adjacent to the interface 18 and isspaced from the first region 20. The second buried pocket 22 alsoextends into the epitaxial layer 16 as suggested in FIG. 1. In thiscase, however, the second buried pocket 22 extends further into theepitaxial layer 16 than does the buried pocket 20. Actually, the drawingdoes not and cannot represent the actual physical structure inasmuch asthere is no distinct boundary between either of the buried pockets 20 or22 and the epitaxial layer 16. However, there are more modifiers in theepitaxial layer 16 which result from out-diffusion and/or autodopingfrom the second buried pocket 22 than from the first buried pocket 20.

In order to accomplish the result just outlined, the two buried pockets20 and 22 are formed by doping the substrate 14 with differentconductivity modifiers. In particular, different elements are used whichmay be, for example, antimony for the first buried pocket 20 and arsenicfor the second buried pocket 22. These two elements are characterized bysubstantially equal diffusion coefficients in silicon, but they havesubstantially different vapor pressures at the temperature of growth ofthe epitaxial layer. The second chemical element, arsenic, has arelatively high vapor pressure at that temperature. Consequently, duringthe formation of the epitaxial layer 16, a portion of that layer whichis adjacent to the second buried pocket 22 will contain an autodopednumber of atoms of arsenic which is greater than the autodoped number ofatoms of antimony in that portion of the epitaxial layer 16 which isadjacent to the first buried pocket 20. There are other combinations ofdifferent chemical elements which might be used in an attempt to provideepitaxial layer portions of different doping concentrations in thismanner, for example, antimony with phosphorus or arsenic withphosphorus. Phosphorus, however, has a relatively high diffusioncoefficient in silicon and it is difficult to control the amount ofphosphorus which enters the epitaxial layer 16. By avoiding the use ofphosphorus and relying on autodoping instead of out-diffusion moreaccurate control can be achieved.

As will appear in the description of the present novel method below, oneadditional difference between the buried pocket 20 and the buried pocket22 is that a higher surface concentration of arsenic is employed for theburied pocket 22 than of antimony for the buried pocket 20. Thissupplements the autodoping effect described above.

The additional elements shown in FIG. 1 are as follows. First, there aregenerally conventional P+ type isolation regions 24 which divide theepitaxial layer 16 into separate islands, 26 and 28 in this example.Means are provided in that portion of the epitaxial layer 16 which isadjacent to the first buried pocket 20, i.e., the island 26, to define acircuit element having relatively high voltage characteristics, inparticular a bipolar transistor. For this purpose, there is a P typeregion 30 adjacent to a surface 32 of the epitaxial layer 16 and an N+type region 34 within the P type region 30. These regions 30 and 34serve as base and emitter regions respectively with the material of theisland 26 serving as the collector region of the transistor. A collectorcontact region 36 may be provided, if desired, adjacent to the surface32 and spaced from the P type region 30.

In the island 28, means are provided to define a circuit element havinga relatively low voltage characteristic, in this example, atwo-collector I² L structure. For this purpose, there is adjacent thesurface of the epitaxial layer 16 a second P type region 38 within theisland 28, and a pair of N+ type regions 40 and 42 within the P typeregion 38, and a third P type region 44 in the island 28 spaced from thesecond P type region 38. Functionally, these regions define a lateralinjector transistor with the region 44 as its emitter, a portion of theisland 28 at its base, and the second P type region 38 at its collector.The switching transistors of the I² L structure are defined by thematerial of the island 28 as the common emitter, the second P typeregion 38 as the common base, and the regions 40 and 42 as thecollectors thereof. An emitter contact region 46 may be providedextending from the surface 32 of the epitaxial layer 16 down to theburied pocket 22. In general this I² L structure is conventional, butdiffers from known structures in that it has better emitter efficiencyowing to the fact that the island 28 is more highly doped than theisland 26, by the use of arsenic as the buried pocket doping materialand the autodoping effect described above.

Some of the steps of the present novel method are illustrated in FIGS.2-4. Conventional steps of etching and cleaning, for example, areomitted in the following description for purpose of clarity.

The process begins with a conventional substrate 14 provided with asurface masking coating 48 of silicon dioxide, for example. By means ofconventional photolithographic procedures, an opening 50 is provided inthe masking coating 48 to define the location of the first buried pocket20. The substrate 14 is then placed in a standard diffusion furnace andantimony is diffused through the opening 50 to provide in the substrate14 a diffusion region 20S of relatively high N type conductivity,labelled N++ in FIG. 2. The diffusion conditions should be chosen so asto provide the region 20S with a sheet resistivity of from 25-30ohms/sq.

With reference to FIG. 3, the masking coating 48 is removed and a newmasking coating 52 is applied to the surface of the substrate 14. Aphotolithographically defined opening 54 is made in the coating 52 atthe location for the second buried pocket 22, after which arsenic isdiffused into the substrate 14 to establish a region 22S. The diffusionconditions for this step should be chosen so as to provide a relativelyhigher degree of doping in the region 22S as compared to the region 20S.A sheet resistivity of around 10 ohms/square is relatively easy toobtain since arsenic has a good lattice match to the silicon of thesubstrate 14.

The next steps are to prepare the substrate 14 in conventional mannerfor the growth of the epitaxial layer 16 and to grow that layer. Theresult is illustrated in FIG. 4 with the diagrammatic showing of thedifferent extents of the regions 20 and 22. Because advantage is heretaken of the autodoping phenomenon, precautions must be taken in thegrowth of the epitaxial layer 16. An important precaution is to employ areactor for the epitaxial deposition in which relatively little lateralflow of gases takes place over the surface of the substrate 14. A knownform of horizontal reactor known as a pancake reactor serves thispurpose well. Next, the growth time and temperature should be controlledso as to provide the epitaxial layer 16 with a thickness of about 6-8μm. This is relatively thin compared to standard practice in the art butallows the layer to be grown relatively quickly and provides a thicknesswhich is particularly advantageous to the I² L section of the device. Ofcourse, during its growth the epitaxial layer 16 may be doped from anexternal source and the desired result is to provide the layer with aresistivity of about 0.5 to about 0.8 ohm-cm. The actual resistivity ofany localized region of the epitaxial layer will depend on thecombination of the external dopant and that resulting from theautodoping phenomenon so that the actual resistivity of the epitaxiallayer is different at different locations in the layer. The resistivityof any localized portion of the epitaxial layer is the sum of thatresulting from the exterior source modifiers and that from the autodopedmodifiers.

By carrying out the process in this manner, it is possible to fabricateboth high voltage and low voltage devices on the same chip, particularlywhere the low voltage device is an I² L device. The high concentrationarsenic buried pocket will autodope the epitaxial region of the I² Ldevice, resulting in better emitter efficiency and higher inverse betaat relatively low breakdown. The low breakdown is not a problem for theI² L section since this portion of a circuit is usually designed tooperate on less than 1 volt of supply voltage. The relatively highvoltage device, such as the transistor described above, can operate atconventional voltage levels.

What is claimed is:
 1. A method of making a semiconductor devicecomprisingforming adjacent to a first portion of a surface of asubstrate of semiconductive material of one type conductivity a firstlocalized region containing opposite type conductivity modifiersconsisting essentially of a first chemical element having apredetermined diffusion coefficient in said semiconductor material and apredetermined vapor pressure at a given temperature, forming adjacent toa second portion of said substrate surface a second localized regioncontaining opposite type conductivity modifiers consisting essentiallyof a second different chemical element having a diffusion coefficientsubstantially the same as said first chemical element but asubstantially higher vapor pressure at said given temperature, growingan epitaxial layer of said semiconductor material on said surface ofsaid substrate at said given temperature whereby that portion of saidepitaxial layer which is adjacent to said second region contains anautodoped number of atoms of said second chemical element which isgreater than the autodoped number of atoms of said first chemicalelement in that portion of said epitaxial layer which is adjacent tosaid first localized region, introducing conductivity modifiers intothat portion of said epitaxial layer which is adjacent to said firstlocalized region to define a circuit element having predeterminedcharacteristics, and introducing conductivity modifiers into thatportion of said epitaxial layer which is adjacent to the secondlocalized region to define a circuit element having characteristicsdifferent from those of the first mentioned circuit element.
 2. A methodof making a semiconductor device as defined in claim 1 wherein saidsemiconductor material is silicon, said one type is P type, saidopposite type is N type, and wherein said first localized region isformed by diffusing antimony into said substrate to a predeterminedsurface concentration and said second localized region is formed bydiffusing arsenic into said substrate to a predetermined surfaceconcentration.
 3. A method of making a semiconductor device as definedin claim 2 wherein arsenic is diffused to a surface concentrationgreater than that of the antimony diffusion.
 4. A method of making asemiconductor device as defined in claim 3 wherein during its growthsaid epitaxial layer is also doped from an exterior source of oppositetype modifiers whereby the number of opposite type modifiers in a givenportion of said epitaxial layer is the sum of the exterior sourcemodifiers and the autodoped modifiers.